A rotary damper disclosed in the Japanese Patent Publication No. 4-34015 is constituted of: a housing; viscous fluid being housed inside the housing; a rotor wherein a resistive portion moving through the viscous fluid inside the housing is provided in an axial portion which is housed inside the housing and whose one part projects from the housing; and a sealing member which prevents the viscous fluid from leaking between the axial portion of the rotor and the housing. A driven gear is attached to the axial portion projecting from the housing.
In the above-mentioned conventional rotary damper, the resistive portion has a roughly oval shape so that the air being mixed into the housing when the rotary damper is assembled is not allowed to be located between the resistive portion of the rotor which is a part of torque occurrence and the bottom face or ceiling face of the housing.
However, since the rotor rotates bi-directionally, when the air being mixed into the housing climbs over the resistive portion and moves to the opposite side of the resistive portion, the air makes a noise.
The noise which occurs when the air being mixed into the housing climbs over the resistive portion is regarded as a plosive sound being caused by the following. When the air being mixed into the housing climbs over the resistive portion after the air being mixed into the housing is compressed by climbing over the resistive portion, the air being mixed into the housing is rapidly opened so that the plosive sound is made.
Incidentally, the higher the degree of viscosity the viscous fluid has, the easier the noise is made. Also, the narrower the distance between the rotor and housing is, the easier the noise is made.
The purpose of the invention is to provide a rotary damper which prevents the noise being caused by the air being mixed into the housing.